The present invention relates to odd order self-corrected electric filters with elements having localized constants of the band-pass type, which make it possible to filter analog or digital signals in this frequency band and which make it possible to correct the distortion of the envelope delay and/or correct the amplitude in said frequency band.
Such filters are particularly useful in radio receivers. In one construction of such receivers, conventional electric filters are used. However, conventional filters do not correct the distortion of the envelope delay and/or amplitude, which is often a disadvantage for the transmission of the signal. The passage time at the different frequencies of the pass band is not constant. Thus, the envelope delay increases from a low value for the centre frequencies to a high value for the limit frequencies of the pass band. The correction function of the envelope delay is brought about by correcting networks. These networks frequently comprise one or more cells incorporating active elements. When there are several cells in the correcting network, each cell is insulated from the adjacent cells by a transistor. These correcting networks are connected in cascade following the filter. In the case of a radio receiver, it is conventional practice to use two cells in cascade for correcting the envelope delay. The centre frequencies of the two cells are positioned on either side of the centre frequency of the band-pass filter which it is desired to compensate. However, the amplitude remains linked with the correction of the envelope delay.
In certain ultra-high frequency applications, microwave filters with a narrow pass band are used. The correction function of the envelope delay is obtained by using coupled multiple cavities. The synthesis of these filters has been described in the technical literature, as is shown by the following publications:
J. D. RHODES "filtre generalise a phase lineaire par cavite a couplage direct", IEEE Transactions MTT, vol. MTT 18, June 1970, pages 308 to 313; PA1 A. E. Atia et al "filtres guides d'ondes a bande etroite", IEEE Transactions MTT, vol. MTT 20, no. 4, April 1972, pages 258 to 264; PA1 A. E. Atia et al "Synthese des cavites multiples couplees a bande etroite", IEEE Transactions CAS, vol. CAS-21, no. 5, September 1974, pages 649 to 655.
The disadvantages of these two types of filters are described hereinafter. Waveguide filters with a narrow band and coupled multiple cavities cannot be used at low or intermediate frequencies for dimensioning reasons and excessive overall dimensions. Coventional filters equipped with correcting networks involve the use of special supply systems for the active cells. In addition, they have particularly large overall dimensions and are not always very reliable.